Thin films of layer-structured perovskite compounds such as SrBi.sub.2 Ta.sub.2 O.sub.9 (SBT), SrBi.sub.2 Nb.sub.2 O.sub.9 (SBN) and their solid-solutions (SrBi.sub.2 (Nb.sub.x Ta.sub.1-x).sub.2 O.sub.9, SBNT) are promising materials for nonvolatile ferroelectric memory applications because of their excellent ferroelectric properties, especially with respect to fatigue performance. .sup.1! The solid solutions, SrBi.sub.2 M.sub.2 O.sub.9 (M; Ta, Nb), known as Aurivillius compounds consist of a stack of alternating layers of (Bi.sub.2 O.sub.2).sup.2+ with two pseudo-perovskite layers of oxygen octahedra, (SrM.sub.2 O.sub.7).sup.2- (M; Ta, Nb) in the c-direction. The high spontaneous polarization is parallel to the plane of the layers (in the a- and b-directions), as this plane contains the O--M--O (M; Ta, Nb) chains that are known to have high polarizibilities as in the perovskite ferroelectrics. It has been suggested that weak fatigue is due to weak pinning of domain boundaries by trapped carriers at shallow Bi and M (M; Ta, Nb) centers. .sup.2!
For these systems, it is imperative that the processing temperatures be lowered below 800.degree. C., since reactions at the bottom electrode-thin film interface cause degradation of electrical properties. .sup.1)-5)
Usually, nonstoichiometric Bi-excess compositions have been used to compensate for the loss of Bi to the substrate during heat treatments. A recent report suggests that fatigue performance is a function of the composition, and nonstoichiometric thin films exhibited low fatigue compared to stoichiometric thin films..sup.3) Thus, if the molecular structure and the stoichiometry of the precursors can be controlled, a stack of alternating layers of (Bi.sub.2 O.sub.2).sup.2+ with two pseudo-perovskite layers of oxygen octahedra (SrM.sub.2 O.sub.7).sup.2- (M; Ta, Nb) in the Aurivillius compounds may be formed at lower temperatures, thereby improving the ferroelectric properties.
To date, there are many reports on the sol-gel method in a broad sense, .sup.3!-8! but only one report discusses the solubility and coordination of bismuth acetate in a solvent. .sup.6! The precursor structure, however, for the layer-structured perovskite thin films has not been reported.
In addition, it has been investigated to make a film thin by the laser abrasion method, the MOCVD method, the sol-gel method and the like; however, they have a problem in common that the processes require a high temperature.
The present inventors studied the molecular structure of the oligomeric precursors for the layer-structured perovskite thin films and the development of the inorganic oxide structure from the precursors, particularly, low-temperature crystallization and the cause thereof. We described the suitable precursor chemistry and the molecular structure of the oligomeric precursors for the layer-structured perovskite thin films elsewhere..sup.4)-10) In this invention, the development of the inorganic oxide structure from the alkoxy-derived precursors is discussed. Specifically, the interrelationship between the precursor chemistry and low-temperature crystallization was determined. Additionally, the ferroelectric properties of stoichiometric thin films are reported.
That is, the present inventors, in order to realize the low-temperature processes, have considered precursor chemistry suitable for low-temperature crystallization and have investigated composite metal alkoxides, and besides, have formed a thin film employing a synthesized composite alkoxide by optimizing precursor solution chemistry, and have investigated the crystallization of the thin film and ferroelectric properties thereof; as a result, they have synthesized a composite alkoxide, have clarified the molecular structure of the composite alkoxide, have controlled the structure to an atomic arrangement similar to that of the sub-lattice of a desired crystal structure, have realized low-temperature crystallization, and have clarified the fact that since the activated energy regarding crystallization of an amorphous gel before a heat treatment prepared by the hydrolysis polycondensation reaction of alkoxide molecules with an atomic arrangement similar to that of crystals is small, as a result, crystallization can be performed at a low temperature, which has led to the accomplishment of the present invention.
That is, it is an objective of the present invention to provide a process for preparing an Sr--Bi--Ta or Nb-based composite alkoxide with the structure of an atomic arrangement controlled.
It is another objective of the present invention to provide a process for preparing a precursor composite alkoxide solution for an Sr--Bi--Ta or Nb-based thin film with the structure of an atomic arrangement controlled to be similar to that of the sub-lattice of the crystal of a desired thin film.
It is still another objective of the present invention to provide a process for preparing a layer-structured perovskite thin film of bismuth-based compouds, employing the above composite alkoxide solution.
It is still another objective of the present invention to provide an Sr--Bi--Ta or Nb-based composite alkoxide with the structure of an atomic arrangement controlled, which is prepared by the above process.
It is still another objective of the present invention to provide a ferroelectric memory element, utilizing the above layer-structured perovskite thin film of bismuth-based compouds.